This invention relates to transceiver circuitry. More particularly, this invention relates to common-mode drivers for use with transceiver circuitry.
It is relatively well known to use transceiver circuitry, which can include transmitter circuitry and receiver circuitry associated with each other, to communicate signals between integrated circuits (“ICs”) or within a single IC. High-speed transceiver circuitry often employs a pair of complementary (“differential”) signals to convey information, where a given bit of information can be indicated by the voltage difference between the two signals. For instance, a logical “0” can be communicated by transmitting and receiving a pair of differential signals where the voltage of the first signal is substantially lower than the voltage of the second signal. Similarly, a logical “1” can be communicated by transmitting and receiving a pair of differential signals where the voltage of the first signal is substantially higher than the voltage of the second signal.
Several problems can exist when using transceiver circuitry. For instance, the common-mode (“CM”) voltage (i.e., the average voltage of a pair of differential signals) of the transmitter circuitry can be substantially different from the CM voltage of the associated receiver circuitry. If the transmitter and receiver circuitries are coupled using a direct-current (“DC”) arrangement, this mismatch of CM voltages can lead to current flowing from one part of the transceiver to another. This current flow can result in a settled CM voltage for the transceiver pair that is substantially different from both the transmitter circuitry's CM voltage and the receiver circuitry's CM voltage. In some cases, the settled CM voltage may be outside the acceptable voltage range of the transmitter circuitry, the receiver circuitry, or both, preventing successful communication between the two parts of the transceiver. In addition, the current flow discussed above can undesirably dissipate power within the system.
As another example, transmitter circuitry implemented with complementary metal oxide semiconductor (“CMOS”) technology often includes at least two current sources, one “P-type” and one “N-type.” If the current sources are of substantially equal strength, the transmitter circuitry's CM voltage can be substantially accurate. However, if the current sources do not substantially match each other, the resulting transmitter CM voltage can deviate substantially from the desired level. This phenomenon is often referred to as “P/N current mismatch.” As discussed above, such deviation can render the transceiver substantially inoperative if the resulting transmitter CM voltage is outside an accepted range, result in undesirable power dissipation in the transceiver, or both. In addition, an inaccurate transmitter CM voltage can result in data errors that can adversely affect the integrity of the system.
As yet another example, there may be a need to generate a step voltage for purposes of receiver detection. For instance, the Peripheral Component Interconnect-Express (“PCI-E”) communication protocol can support receiver detection by generating a transmission voltage that changes from one level to another, and measuring whether a transmitted signal changes relatively slowly or relatively quickly. If a functional receiver is coupled to the transmitted voltage signal, the delay resulting from the transmitter's termination resistance, the receiver's termination resistance, and the DC circuitry's capacitance (collectively, the “RC delay”) can cause the transmitted signal to change relatively slowly. On the other hand, if no receiver is coupled to the transmitted voltage signal, the signal can change relatively quickly.
In view of the foregoing, it would be desirable to provide methods and apparatus that can selectively set a CM voltage for a transceiver, reduce the effect of current mismatch, and generate a voltage step that can be used for receiver detection. It would further be desirable to provide a single solution that can address all of the concerns discussed above.